1. Field of the Invention
The present invention relates to measuring color. More particularly the present invention is a colorimeter based on the standards for color established by the International Committee on Illumination. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Accurate color measurement, or colorimetry, is needed in many fields of industry, technology and art. For instance, accurate color matching is vital in assuring consistent batch-to-batch colors in making paints, dyed goods and metal alloys. Classical matching "by eye" is inaccurate and, therefore, inadequate. In determining the color of alloys used in jewelry making, for example, alloys of similar elemental composition may not be similar in color, while a good color match can often be obtained between quite different alloys; the same is true of paint pigments, dyes, or virtually any other product whose color is of any importance.
A mathematical system of colorimetry has been developed by the International Committee on Illumination (C.I.E.) and is widely used as a standard. However, instruments for measuring color according to this and other formal standards are prohibitively expensive for most applications.
The C.I.E. system begins by quantifying the human eye's response to color and luminosity. Three "tristimulus" functions X, Y and Z are defined to correspond, not to specific wavelengths of light, but to the approximate sensitivity of each of the three types of human retinal cone cells to varying wavelengths. Because sensitivity varies from one individual to another, and also from time to time in one individual, the X, Y and Z functions necessarily represent compromises defined for mathematical convenience. For a discussion of how the tristimulus functions are defined, one is referred to the publications of the Bureau Central de la C.I.E. in Paris, France.
To identify a specific color, as perceived by the average human eye, it is both necessary and sufficient to specify numeric values for X, Y and Z. These may then be transformed mathematically into the more familiar quantities of hue, brightness and saturation, if desired.
A moderately close approximation to X, Y or Z for a given light source can be obtained by passing light from the source through a red, green or blue filter whose transmission spectrum approximates X, Y, or Z, respectively, and then into a light-detecting cell whose response is nearly uniform across the visible spectrum. Filters may be dye, glass, liquid solutions, or a combination. However, no known combinations precisely match the X, Y and Z tristimulus functions. Hence, use of filters will give only an approximation to X, Y, and Z for a given source.
Much better accuracy is given by what is called a "template" colorimeter. In such a device, light from a source is dispersed according to wavelength, using a prism or diffraction grating, and projected onto templates which are cut out in the forms of the X, Y and Z tristimulus curves. The light passing through the templates then falls on light-detecting cells.
While the template method makes it possible to match the tristimulus curves to any desired degree of precision, the added optics and the need to change the templates and register their positions accurately make the device complex, cumbersome and expensive.
A more modern approach to "template" colorimetry could be implemented using a diode-array spectrophotometer with a built-in computer, such as the Hewlett-Packard 85. Light intensities at a large number of wavelengths could be measured, digitized, stored, and then multiplied by tabulated tristimulus values (such as those in section 9 of the Optical Society of America's Handbook of Optics) and summed to give the values of X, Y and Z. While well-suited to laboratory use, this approach is hardly practical for the occasional user, such as a small businessman or jeweler, because of cost and the need for custom programming to carry out this function. Color television technology could be used to give a similar result, but would be similarly expensive and would give a generally poorer match to the tristimulus curves, likely varying in quality depending on the source of the equipment.
To make accurate and universal C.I.E. color standardization an affordable, accurate reality, some measurement means will be needed which combines the high accuracy of the template and spectrophotometer methods with the low cost and simplicity of a filter-based device. No such means is presently available.